Cant make glare work while compositing
Try lowering the Threshold value.
The Glare node works based on the luminance (or brighness) of the pixels in the image, the Threshold value means that pixels that are brighter than that number will be affected by the glow, while values lower than the set number will not.

By lowering it, you are changing the node's level of tolerance. If it's set to a low value, it means that even a small amount of light will make the glare show up.
You also may be rendering in EEVEE. Switch to Cycles.